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Diagenetic reorientation of phyllosilicate minerals in Paleogene mudstones of the Podhale Basin, southern Poland

Lookup NU author(s): Ruarri Day-Stirrat, Professor Andrew Aplin


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We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60-160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 μm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40-50% illite layers changes to R1 illite-smectite with 70-80% illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change. Copyright © 2008, The Clay Minerals Society.

Publication metadata

Author(s): Day-Stirrat RJ, Aplin AC, Środoń J, van der Pluum B

Publication type: Article

Publication status: Published

Journal: Clays and Clay Minerals

Year: 2008

Volume: 56

Issue: 1

Pages: 100-111

ISSN (print): 0009-8604

ISSN (electronic): 1552-8367

Publisher: The Clay Minerals Society


DOI: 10.1346/CCMN.2008.0560109


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